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We know if we take immiscible liquids of different densities then they form different layers on earth. But when we take them to outer space, they form tiny droplets which are energetically most favourable for them. This is because there is no gravity, so no preferred direction for the liquids to settle.

But if we take miscible liquids of different densities, then at least on earth they form different levels (salt water, fresh water). Although there is a mixing between the two layers, that is a fairly slow process.

My question is what would happen if we take a container containing unmixed (in two seperate layers) miscible liquids of different densities and switch gravity off. I think there would be mixing in this case too. But would it be faster or slower?

I think if the liquids are already mixed then there won't be any considerable change in their configuration.

It would be better if the answers do not have a very high mathematical rigour.

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The slow mixing you are referring to, is called diffusion. On the microscopic level, the molecules move in random direction, this is called Brownian motion. Without gravity, this microscopic motion is still present (unless you go to the absolute zero temperature).

With gravity (suppose you have the heavier layer on the bottom), this is an unmixing force, which slows down the mixing process (or increases separation). So without gravity, this diffusion is the only process playing a roll, and will thus be faster. (If you put the heavier phase on top with gravity, you will get all kinds of interesting instability phenomena).

As an useful analogy you could consider temperature diffusion in solid materials. This is obviously not affected by gravity. Putting two layers of different temperature together, will eventually 'mix' the temperature, such that it is uniform.

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  • $\begingroup$ I don't think I get your point about the heavier phase being on top. In presence of gravity with the heavy phase on top, the two layers would naturally mix as the heavier comes down and the lighter goes up and this should make the process faster, right? So without gravity shouldn't it be slower? $\endgroup$
    – Normie
    Commented Oct 10, 2015 at 10:14
  • $\begingroup$ Well, with the heavier phase on top it might first mix, but in the end the heavy phase will all be at the bottom (if gravity is stronger than difussion). So it may first seems to mix, but then it will separate again. $\endgroup$
    – Bernhard
    Commented Oct 10, 2015 at 11:09
  • $\begingroup$ But miscible liquids once mixed would not seperate again. So why is it not probable that the heavier liquid being on top first goes down and the lighter one rises. In this process the mixing happens fast enough that the heavier one doesn't get enough time to settle down. How can one definitely comment that the mixing will be slower without knowing the density and concentration ? $\endgroup$
    – Normie
    Commented Oct 11, 2015 at 10:25
  • $\begingroup$ Argh, I see made a crucial error in my answer (see my edit). With the heavy phase on top you may be interestedi n Rayleigh-Taylor instabilities (en.wikipedia.org/wiki/Rayleigh%E2%80%93Taylor_instability). Indeed, with the heavier phase you get quick mixing due to convection. Density and diffusion plays an important role, not so much concentration. $\endgroup$
    – Bernhard
    Commented Oct 11, 2015 at 10:44
  • $\begingroup$ The Rayleigh Taylor instabilities hold for immiscible liquids it seems according to the wiki article. $\endgroup$
    – Normie
    Commented Oct 11, 2015 at 10:51
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Assuming the fluids completely fill the closed container and the temperature is not near zero, nothing noticeable would happen. Each molecule in the fluids experience three basic types of forces. One is simply the force due to the net electrostatic potential between each molecule. For example Van der Waals. Another is simple collisions. The third is gravitational interactions (earth and other molecules). Gravity is by far the smallest of these forces. The reason is the mass is so small and the velocities are so high.

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  • $\begingroup$ Not true, gravity affects the liquid layers in earth more than the electrostatic interactions. That is why we find different layers. $\endgroup$
    – Normie
    Commented Oct 11, 2015 at 10:27
  • $\begingroup$ The reason you see different layers is due to the relative surface tension between the two fluids and has nothing to do with the relative densities. For example, water boils at 100C, not because it overcomes gravity, but because it's vapor pressure equals that of the atmosphere. At 1 atm, water boils at 100C on earth and in space. $\endgroup$
    – gogators
    Commented Oct 13, 2015 at 9:48
  • $\begingroup$ Relative surface tension would not create a directionality of seperation. The fact that we see a directionality with oil on top and water down is because there is gravity. $\endgroup$
    – Normie
    Commented Oct 13, 2015 at 21:08
  • $\begingroup$ Yes, you are correct. More precisely, the gravity is why the interface between the fluids is a plane normal to the gravitational gradient. But gravity is not why you have different layers in the first place. If the fluids did not experience gravity and were not in a container, they would still form layers, but spherically. But your question was what if they were in a container and you turned gravity off. $\endgroup$
    – gogators
    Commented Oct 13, 2015 at 21:16

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